1. Field of the Invention
The present invention relates to a method of making synthetic silica glass, more specifically to a method for doping chlorine into silica glass.
2. Description of the Background Art
It is known that the refractive index of silica glass may be increased by doping oxides such as GeO.sub.2 or TiO.sub.2. Chlorine is another dopant that increases the refractive index. As shown in the literature "Reduction of Chlorine Content in Sol-Gel Derived Silica Glass" by K. Susa et al. in J. Non-Crystalline Solid, Vol. 79, 1986, pp. 165-176, chlorine can increase the relative refractive index difference by 0.11% per 1 weight % of chlorine. However, the sol-gel method used in this literature has difficulty in producing a large preform. And unexamined published Japanese patent application Tokukaihei 3-115136 discloses that the refractive index may be controlled by changing the chlorine partial pressure during the treatment of a deposit of glass particles called "soot," which is formed by flame hydrolysis, in a mixed atmosphere including chlorine gas. However, this method controls the relative refractive index difference only by about 0.04%. Unexamined published Japanese patent application Tokukaishou 63-315531 discloses a heat treatment in an atmosphere including SiCl.sub.4. However, SiCl.sub.4 is used for a different purpose and it is not disclosed to add chlorine into silica glass.